Ed Miliband led on how welfare reform will affect cancer patients at PMQs this week. It was a good decision. It's an important issue. And it's an issue you can't play politics with.

There was plenty of potential for David Cameron – a big, succulent mud pie of Tony Blair, Liam Byrne, the Volvo plot, David Miliband's leaked speech and male ostriches…

But Ed Miliband refused to play that game. He couldn't afford to get hit by something dirty.

Instead, he went on an emotive subject that demanded a serious debate.

The Labour leader quoted figures from Macmillan Cancer Support that suggest almost 7,000 cancer patients could lose up to £94 a week under changes set out in the Welfare Reform Bill. The legislation is being voted on in the Commons this evening.

David Cameron responded that the figures were "simply not true". (One PM source after PMQs suggested that Macmillan's figures were "guesswork".)

"He doesn't know the detail of his own Bill," Ed Miliband said. "Many people will lose their benefits simply because they don't recover quickly enough."

Macmillan Cancer Support have said that plans to limit the Employment and Support Allowance to one year will have a "devastating impact on many cancer patients".

"He's wrong. He should admit he's wrong," Cameron retorted, shaking his head. "He wants a distraction from why he doesn't support the bill."

Miliband wasn't taking that. "Why doesn't he know about these arguments? The House of Commons is voting on this tonight."

"He's attempting to put up a smokescreen," the prime minister responded. "A weak leader of a divided party. That's what we've had this week."

He finished with a couple of digs about Tony Blair and David Miliband. But, by this stage, both sides of the House were shouting so loudly that Cameron's sharpest response was barely audible in the chamber. Speaker Bercow didn't try to shut them up.

It's a "disgrace" to say talking about cancer patients is a smokescreen, Miliband stated. After the session, Labour sources called on the prime minister to apologise for the comment.

It's been a challenging few days for Ed Miliband. But the prime minister found it difficult to chime with the mood of the debate. And Labour seized upon it.

Theatrics, put-downs, witty retorts – this is what we expect from PMQs. Cameron is comfortable with this format. Ed is not yet.

But this week's exchange was serious and policy-led. This was Ed Miliband's comfort zone and he was bolstered by setting this tone.

He won't always be able to avoid getting mucky. But Miliband won't have to hang out any more dirty washing as a result of this PMQs.

Tags: David Cameron, Ed Miliband, PMQs